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  • For Safety’s Sake: single line drawing

Whether a building is old or new, electrical engineers depend on single-line diagrams (SLDs) to track the electrical components that assure proper maintenance and safety practices. Serving as a map of the electrical distribution system in a facility, an SLD documents conductors, transformers, overcurrent protective devices (OCPDs) and other electrical devices and safety mechanisms to aid in many areas of design and maintenance while reducing confusion.

An SLD is a primary resource to calculate short-circuit currents, determine selective coordination and, ultimately, calculate incident energy—making it one of the most important safety documents at a facility’s disposal. While critical to the safe operation of a facility, SLDs all too often don’t get the attention they warrant.

Why an updated SLD is important

Early in my career I performed a short-circuit study for a large industrial facility. Their SLD drawings referenced a motor control center (MCC) that couldn’t be found during my walkthrough. An on-site electrician told me the MCC had been removed years ago, then pointed out the date on the SLD. The documents were more than 10 years old with no updates recorded. To begin my study, I had to re-draw the entire power distribution system—a time-consuming and costly endeavor for the facility.

Beyond the chore of having to redraw the SLD, the documents lacked the accurate information needed to assure that safety protocols, such as lock-out tag-out, could be put in place to prevent work on energized equipment. In my experience, a laissez faire attitude toward SLD procedures increases the chance of a serious electrical accident.

An SLD is a primary resource to calculate short circuit currents, determine selective coordination and calculate incident energy—making it one of the most important safety documents at a facility’s disposal.

Thomas Domitrovich, vice president, technical sales

Creating and maintaining an SLD

When a building is first constructed, electrical engineers and contractors work together to create an SLD via a standard process:

  • A design engineer lays out the components and devices to be installed.
  • The engineer creates assumptions used to estimated fault-current calculations, equipment evaluation and selective coordination needs.
  •  A contractor then performs installation, marking-up the SLD as changes are made. The contractor often adds lengths of conductors and accounts for transformer and motor/ generator nameplate data, including impedances, as power generation additions can increase fault current within a system.
  •  As-built drawings (the original design drawings revised to reflect changes made in the field) are created, complete with updated fault-current calculations.

Once a facility opens, all changes are reflected on an SLD, with documentation reviews every five years to assure accuracy and clarity.


Complete and maintain SLDs according to industry guidelines

Various sections within NFPA (National Fire Protection Association) 70B and 70E suggest or mandate SLD creation and updates. In Section 6.2.2.1, NFPA 70B recommends that SLDs show all electrical equipment in the power system and give all pertinent ratings for voltage, frequency, transformer impedance, available short-circuit current and OCPDs, among others. Further, as stated under NFPA 70E Section 130.5(G), “The incident energy analysis shall be updated when changes occur in the electrical distribution system that could affect the results of the analysis. The incident energy analysis shall also be reviewed for accuracy at intervals not to exceed five years.”

But, even with clear NFPA guidelines and creation and maintenance processes, SLDs frequently go unfinished after a new build. Construction managers often aren’t trained to understand the importance of SLDs and the calculations that stem from the fault-current values they contain. Further, SLDs are rarely reviewed and updated over the life of a facility. Management and maintenance professionals don’t use SLDs on a daily, monthly or even yearly basis. So, there’s a risk that the documentation is placed in the recesses of storage or the back of a filing cabinet and forgotten.  


Keep your SLD current and plan for updates

Electricians and design professionals rely on accurate SLDs to calculate the short-circuit values that ultimately determine incident energy. So, do your due diligence by preparing and maintaining SLD documentation. If you’ve made infrastructure changes to your electrical system, make sure your drawings are updated. And remember to review your SLDs every five years per NFPA 70E, whether you’ve made changes or not.

Additionally, I strongly recommend creating an SLD budget line item to account for the resources needed to record changes and conduct the five-year documentation review. The industrial facility example I referenced earlier highlights this importance. If the company had updated its diagrams over the years, I would have simply performed short-circuit and coordination studies. Instead, I spent hours diagramming wiring before I could perform the original scope of work, which was an unplanned and costly expense. In my experience, spending some money on proper documentation in the near term lessens the chance of a much larger service expense later on down the line.  

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